1. Field of the Invention
The present invention relates to an exposure head, and in particular, to an exposure head which is used in an image exposure device or an image recording device or the like, and which can project, by a projection lens, a light beam from a suitable light source.
2. Description of the Related Art
Digital-exposure-type exposure devices are known in which a light beam, which has been modulated in accordance with image data, exposes a photosensitive material in order to record an image. In order to shorten the recording time, many plural-light-beam optical systems have been proposed in which a plurality of light beams substantially simultaneously expose a photosensitive material. In this case, when the optical system is structured by using plural light sources to obtain plural light beams, variations between light sources and assembly errors in the respective light sources lead to offset of pixels and non-uniformity of spot configurations due to positional errors of the respective light sources. Non-uniform density is caused in the obtained image, which results in a deterioration in image quality.
In order to overcome this drawback, techniques have been proposed for providing apertures at the light-exiting sides of the light sources in order to make the light spot configurations uniform, and for providing diffusion plates at the light-exiting sides of the light sources in order to make the light amounts uniform (see Japanese Patent No. 2771932 and U.S. Pat. No. 4,999,648). In these techniques, apertures of the same opening diameter are provided at the same pitch at the light-exiting sides of the plural light sources. These apertures are provided at the conjugate position with respect to the position of a photosensitive material. The light beams from the light sources which have been collimated onto the apertures are illuminated onto the photosensitive material, and the diffusion plates are provided in vicinities of the apertures. In this way, light beams, whose configurations and profiles have been made uniform, can be illuminated on the photosensitive material.
However, in these conventional techniques, although simply providing apertures or providing diffusion plates makes the spot configurations homogeneous to a certain extent, these techniques are not suitable for finely homogenizing the spot configurations because of the locality in the degree of diffusion of the diffusion plates.
Further, for example, it is difficult to fit a light source and an aperture, or an aperture and a diffusion plate closely together. Thus, positional errors arise in accordance with the distance between the light source and the aperture, or the distance between the aperture and the diffusion plate.
When diffusion plates are used, if the degree of diffusion thereof is low, at elements other than those on the optical axis of the focusing lens, the LED image and the two-dimensional light source image on the diffusion plate will be focused such that they are offset from one another, which leads to variation in the configuration.
In view of the aforementioned, an object of the present invention is to provide an exposure head in which a deterioration in image quality, such as non-uniform density or the like, in an image formed by using plural light beams can be suppressed.
In order to achieve the above object, an aspect of the present invention is an exposure head which projects a light beam from a light source onto a photosensitive material by a lens in an exposure device which exposes the photosensitive material, the exposure head comprising: the light source formed by a plurality of light emitting elements; and a lens array having lenses of a number corresponding to a number of the plurality of light emitting elements.
In the above-described exposure head, a lens array is provided which has lenses of a number corresponding to the number of light emitting elements, the lenses being provided at the light-exiting side of a light source in which a plurality of light emitting elements are arranged, for example, linearly. A microlens array in which a plurality of microlenses are arranged in an array form may be used as the lens array. In this way, even in a case in which the light beams from the light source are not uniform, light beams from the light source can be made uniform by the respective lenses of the lens array. Further, the diffusibility of the light at the light-exiting side of the lens array can be improved by the light collecting property of the lenses.
The lens array may be structured such that the configuration of the light-incident surface and the light-exiting surface of each of the lenses is convex. By increasing the distance between the light-incident surface and the light-exiting surface of each lens in such a lens array, each lens can function as a lens group in which two lenses are combined. An example of a lens in which the configuration of the light-incident surface and the configuration of the light-exiting surface are convex is a convex lens. By making each lens a lens in which the boundary surface configuration of the lens is convex (e.g., by making each lens a convex lens), for each of the lenses, the light beam can be collected along the optical axis.
Further, a plurality of lens arrays can be disposed in the optical axis direction. Namely, the light beams emitted from the light source can be collected by dividing the collecting functions up amongst the plurality of lenses. For example, a structure in which two lens arrays are disposed in the optical axis direction can be considered to be a structure corresponding to a so-called illumination-type optical system. In this case, the lenses (lens array) at the light source side corresponds to collector lenses, and the lenses (lens array) at the exposure side correspond to condenser lenses. By providing two lens arrays in this way, the optical settings relating to the light beams from the light source can be achieved easily.
Each lens of the lens array may be a lens having positive power, i.e., a convex lens.
The exposure head of the present invention may be provided with an aperture device in which a plurality of apertures are formed at uniform intervals, the number of the apertures corresponding to the number of the plurality of light emitting elements. The light beams passing through the respective lenses are limited by the apertures of the aperture device. Namely, the transmission of light beams at regions other than the periphery of the optical axis or on the optical axis of the lens can be suppressed.
The diameter of each aperture of the aperture device may be less than or equal to the effective diameter of the lens. In this way, only the light beams passing through the respective lenses are effectively limited.
The aperture device may be provided between the light-incident surfaces and the light-exiting surfaces of the respective lenses. In this way, only the light beams passing between the light-incident surface and the light-exiting surface of each lens are limited, and only the light beams passing through the apertures exit.
The exposure head of the present invention may further include a diffusing device for diffusing the light beams exiting from the lens array. The light beams exiting from the lens array can be efficiently diffused by the diffusing device.
The lens array may have a number of lenses which corresponds to a number which is greater, by an even number, than the number of the plurality of light emitting elements. For example, the same number of lenses may be provided at each side of the plurality of lenses corresponding to the plurality of light emitting elements. In this way, the same state can be obtained for each of the light emitting elements. Namely, for each of the light emitting elements, the light emitted from the light emitting element is incident on the corresponding lens as well as on the lenses adjacent to that lens. Namely, the light from the light emitting elements disposed end portions as well as the light from the light emitting elements disposed other than the end portions can be incident on the lenses adjacent to the corresponding lens as well as on the corresponding lens. The states of the obtained light beams can be made homogeneous.